This invention relates to magnetic recording media and more particularly to a binder system for rigid storage media.
As magnetic recording densities of rigid storage media progressively increase and track widths correspondingly decrease, magnetic bit cells are of such size that the use of hard non-magnetic particles, which have been commonly used to improve the wear resistance of disk coatings, must be minimized. Further, as new magnetic particles such as cobalt surface diffused or cobalt modified .gamma.-Fe.sub.2 O.sub.3 magnetic particles are developed to meet density requirements, high matrix curing temperatures have been found to impair the particle magnetic properties. This degradation may occur as a consequence of cobalt diffusion and reduction of magnetic properties. Also, the coating material must be capable of forming a very thin coating with a smooth surface that will allow a transducer carrying slider assembly to fly only a few millionths of an inch above the surface. These properties are in addition to the commonly required characteristics of coatings for rigid magnetic disks such as being adaptable to normal coating techniques, capable of withstanding repeated head loadings, able to achieve high orientation ratios and capable of good abrasion resistance, impact resistance and corrosion resistance.
All these conditions require a suitable highly cross-linked polymer material which has thermosetting polymer properties such as hardness, toughness, insoluability and infusibility. Although thermoplastic polyurethane magnetic coatings are well known for producing magnetic tape or flexible magnetic disks. In those applications the supporting substrates are plastic materials such as mylar. Mylar substrate materials deform at temperatures above 120.degree. C., and it is known that two component polyurethane material may be cured at temperatures below 120.degree. C. Flexible media systems of this type are shown in the U.S. Pat. Nos. 4,058,646; 4,068,040; 4,152,485; 4,328,282 and 4,333,988. It is also known that existing two component polyurethane systems have a short pot life, from 4 to 8 hours, which causes problems with regard to magnetic particle dispersion. This problem of limited pot life is addressed by the teaching of U.S. Pat. No. 4,235,766 which provides the improvement of a pot life that is extended to 8 to 16 hours to enable better dispersion of magnetic particles within the coating. Mere extension of pot life is not an adequate solution since the physical characteristics are constantly changing during the period. It is necessary to use the coating material quickly during the same period in the pot life to assure a uniform repeatable process and resultant coating. This is difficult to achieve with tape or flexible disk materials wherein large quantities are processed rapidly and thereafter cut into tape or blanked into disk form. In the coating of rigid disk substrates that are individually coating, it is not practical to use a coating material having a pot life of less than 500 hours. The best resolution of the pot life problem is to obtain a material that has an infinite pot life so that the production conditions such as coating rigid disk substrates can be uniformly reproduced.
Rigid disk media differ from flexible media not only with respect to magnetic particle orientation problems, but also as a result of the coating using a rigid thermosetting binder system that has difficulty adhering to the substrate. In addition, the flexible polyurethane materials do not withstand the long term head loading under normal hard disk speeds of 3600 revolutions per minute. In flexible disk applications rotation speeds are normally less than 500 revolutions per minute.
Thermosetting polyurethane materials are known to have several magnitudes higher wear resistance than thermoplastic material. However, rigid magnetic media are formed by applying the coating on a nonmagnetic substrate, commonly aluminum. The aluminum substrate exhibits poor adhesion to conventional thermosetting polyurethane which precluded use of the binder system in normal disk making. Conventional thermosetting polyurethane also exhibits poor adhesion to the mylar substrate of the flexible systems and use of a rigid form of that material would not alleviate the problem of bonding the coating.